Query Regarding Unexpected Results in Colloidal Fe and As Simulation

[Sen]

Dear Dr. Parkhurst，
I hope this message finds you well. I am currently working on a simulation involving the formation of colloidal Fe and As, but I have encountered an issue. According to my calculation results, the concentration of As at equilibrium is unexpectedly higher than the initial concentration in the solution. Could you kindly take a look at my code and help identify any possible errors? Please see Coll.Fe-As01.xlsx
Thank you for your assistance!
Best regards,
Sen

Code: [Select]

PHASES
Fix_H+    # Define pH fixing mineral
H+ = H+
log_k 0   # pH = 10^(-SI)


SELECTED_OUTPUT
-file                Coll.Fe-As01.xlsx
-user_punch          true
-high_precision      false
-ph                  true
-pe                  true
#-reaction            false
#-temperature         false
#-alkalinity          false
-totals              As(5)  As(3)     Fe(3)      Fe(2)
-molalities          Hfo_wH2AsO4   Hfo_wHAsO4-   Hfo_wOHAsO4-3   Hfo_wH2AsO3    Hfo_wCO3-   Hfo_wHCO3
-equilibrium_phases   Fe(OH)3(a)                     

#background electrolyte
SOLUTION 1
    temp        25
    pH          2
    pe          4
    redox       pe
    units       mol/l
    density     1.0     
    Na          5.0e-3
    Cl          5.0e-3
    As(5)       1.78e-03   
    C(4)        1.05e-5
    Fe(2)       1.8e-4
  SAVE solution 1
  END


SURFACE 1              # define small conc's
-equilibrate with solution 1
Hfo_w      2.4e-3    600    0.01   Dw 1e-11
Hfo_s      6e-5
-Donnan     1e-12
-equil 1

SAVE solution 1
SAVE surface 1
END


USE solution 1
USE surface 1
EQUILIBRIUM_PHASES 1
        O2(g)      -39
     -force_equality   true
     #   CO2(g)     -3.73   
     #-force_equality   true
        Fix_H+     -7.0   NaOH  0.1
     -force_equality   true

As_native     0 0
As2O5(cr)         0 0
Arsenolite        0 0         #As2O3
Claudetite        0 0         #As2O3
Fe(OH)3(a)        0 0

END



[dlparkhurst]

No Dr.

You have defined a solution and equilibrated a surface with that solution. If you simply reacted the solution with the surface, you would have little reaction (except possibly for redox reactions between Fe(2) and As(5). However you changed the pH, which creates a new equilibrium between the surface and the solution. Some of the As on the surface partitions from the surface to the solution creating a higher arsenic concentration in solution.